1. Field of the Invention
This invention pertains to the field of power transmission systems for vehicles, and more particularly, to such systems that transmit power at the option of the vehicle operator either to a first set of drive wheels or to first and second sets of drive wheels.
2. Description of the Prior Art
Formerly, in systems that provided only part-time four-wheel drive, a vehicle operator had to exit the passenger compartment in order to set the front wheel hub locks for four-wheel drive operation, then re-enter the vehicle and move the shift selector that controls the transfer case to the four-wheel drive position. Later, manufacturers introduced full-time, four-wheel drive systems that caused the front drive components to rotate continuously, thereby avoiding the need for selectively operated hub locks. Full-time four-wheel drive systems, however, have substantially reduced fuel economy compared to two-wheel drive systems because a large inertia mass must be continuously rotated.
Recently, automatic hub locks were introduced to eliminate the need for the operator to exit the vehicle in order to manually engage hub locks on the axle that is driven only part of the time. In some of the part-time, four-wheel drive systems, the vehicle must be stopped and shifted to four-wheel drive; then automatic hub locks are engaged when the vehicle is driven ahead. Hub lock disengagement is accomplished the first time the vehicle is moved in the opposite direction provided the transfer case shift is in the two-wheel drive position.
Still more recently, drive systems have been manufactured that permit shifting from two-wheel drive to four-wheel drive while the vehicle is moving. In such systems when two-wheel drive is selected, a sliding synchronizer collar within the front axle assembly disconnects one of the front axleshafts from the front differential allowing the ring gear of the differential, the front driveshaft and the transfer case chain to remain stationary while operating in two-wheel drive. However, these systems require that the casing, pinion and gears of the differential and both front axleshafts be driven by the front wheels during two-wheel drive operation, so loss of efficiency and reduced fuel mileage result. To engage four-wheel drive, the operator moves a shift lever to the 4-high position, which action energizes the synchronizer in the transfer case.
The sliding collar within the axle assembly disconnects one axleshaft from the differential so that the ring gear of the differential, the driveshaft, and the transfer case chain remain stationary while the system produces two-wheel drive. However, the pinions and gears of the front differential unit and the front axleshafts rotate with and are driven by the front wheels during two-wheel drive operation. In order to engage four-wheel drive, the operator moves the shift lever to the four-wheel drive high speed ratio position. This action energizes the transfer case synchronizer to driveably connect the front drive shaft and causes it to accelerate to the speed that corresponds to the vehicle speed. Then a vacuum valve in the transfer case activates a vacuum diaphram mounted in the engine compartment. The diaphram pulls a cable, which moves the sliding collar in a front axle assembly to connect the differential to the previously disconnected front axleshaft. The shift from two-wheel drive to four-wheel drive can be made at speeds of up to 55 mph, but during cold weather and under other adverse conditions, where transmission oil viscosity is a factor, the shift must be made at slower speeds in order to keep the effort required to move the shift lever at a reasonable magnitude. In four-wheel drive operation, shifts are made between the high speed ratio and low speed ratio provided the vehicle is stopped and the transmission is first shifted to neutral, as is the conventional practice.
In a part-time four-wheel drive system, conventional manual or automatic locking hubs may be used in connection with the front wheels. When the hubs are unlocked, the front wheels can rotate free of the front drive mechanism. In addition, the chain and sprocket assembly of the transfer case is disengaged by moving a shift lever in order to stop the front-drive mechanism.